DE-A-0 342 651 B1 describes a manually operable metering pump with the characteristics contained in the precharacterising part of claim 1. The ability, provided by standard pumps of this known type, to ventilate a container equipped with such a pump encounters difficulties in those cases where the medium that is contained in the container and that is to be dispensed is highly viscous, such as e.g. creams, and is to be prevented from contacting environmental air so as to prevent loss of function of the pump and contamination of the medium by harmful germs or dirt particles contained in the air.